Antarctic Mass Balance and Sea Level Change

The Antarctic ice, remote as it is, has the potential to affect coastal communities and habitats around the world, because any change in its mass will directly affect mean sea level. Despite all the study of the ice sheet in the last several decades, however, it is still uncertain whether the ice sheet is growing, shrinking, or unchanging in mass. The most recent studies are contradictory-glaciological evidence from the continent suggests that the ice sheet is slowly growing, oceanographic evidence around the continent suggests that it is shrinking, and indirect satellite evidence relating to changes in mass distribution over the surface of the globe suggests that changes of either sign must be small.

The glaciological evidence has most recently been summarized by Bentley and Giovinetto [1991]. By analyzing all available measurements on the rate of snow accumulation on the ice sheet (the mass input) and the mass flux across the boundary between the land-based and floating margins of the ice sheet (the mass output), they came to the conclusion that there is a net gain of about 200 Gt/yr, that is, a sea-level lowering of about 0.5 mm/yr. The data cover the large drainage systems that encompass the vast interior plateaus where the mass input rates are small. Much less is known about more limited coastal systems characterized by higher specific mass input rates. Nevertheless, it seems unlikely that these smaller regions could be losing mass rapidly enough to make up for the mass gain over the majority of the continent.

Oceanographic studies tend to contradict this result. Jacobs et al. [1992] and Jacobs and Hellmer [1992] used measurements of temperature, salinity, and currents along with oxygen isotope ratios in the ocean in front of the great Antarctic ice shelves to estimate the rate of melting from the under sides of the shelves. Combining the mass loss with estimates of iceberg flux, they calculated a total mass loss for the entire ice sheet of about 500 Gt/yr. Calving of icebergs from floating ice fronts and melting under the larger ice shelves are the only important modes of ice loss from the ice sheet. Melting from the surface of the ice is negligible because temperatures almost everywhere in Antarctica remain below freezing even in mid-summer. The latest estimates of iceberg flux have brought more disagreement between the oceanic and continental measurements. Orheim [1993] analyzed over a decade of shipboard observations that suggest that his earlier estimate may be too small by 1000 Gt/yr or more.

The mass losses indicated by oceanographic measurements cannot be translated directly into changes in sea level, however, because they represent losses from the ice shelves, which are already afloat. Thus, the continental and oceanographic measurements could be reconciled if there is sufficient ongoing shrinking in the size of the ice shelves, which extend around most of the Antarctic coastline. There is abundant evidence from satellite imagery that the fronts of some of the ice shelves are indeed retreating, but whether at the large rate of about 1 km/yr, which is required to produce balance, is problematical. The U.S. Geological Survey is investigating this.

In an entirely different approach, Trupin [1993] has shown that the rates of change in the low-order zonal harmonics - the coefficients in the Earth's gravitational potential that primarily represent the broad change in mass distribution in the north and south polar parts of the planet - of the Earth's gravity field to be expected from any large redistribution of mass from the south polar region to the world ocean would be substantially larger than the rates of change that have been found from analyses of satellite orbits. This would seem to limit any mass imbalance to no more than a few tenths of a millimeter per year.

Antarctic glaciologists are dissatisfied with this situation. The Scientific Committee for Antarctic Research, the committee of ICSU that coordinates national Antarctic programs, has established a Group of Specialists on Global Change and the Antarctic, with a subgroup on Antarctic Mass Balance and Sea Level. In September the subgroup, which comprises glaciologists from countries with active Antarctic research programs, met to plan a campaign to extend the measurement base to the many unsurveyed parts of the ice sheet, taking advantage of the powerful capabilities of new satellite techniques. Perhaps the answer to the mass-balance conundrum will be known by the end of the millennium.

It is a surprising fact, in light of the uncertainties about the present situation, that at least for the next century or two, the effect of climate change on Antarctic mass balance is easier to estimate than the current balance because only the mass input will change over that time scale. Significant changes in the huge ice sheet's rates of ice flow can occur only after delay times measured in thousands of years. A variety of studies, both observational and theoretical, are in fairly good agreement that there will be an increase in the mass input to the ice sheet of about 200 Gt/yr1, corresponding to a sea-level lowering of 0.5 mm/yr, for each degree of climatic warming [Bentley and Giovinetto, 1992]. That is because warmer air can carry more moisture, and most of the moisture carried in air masses that move over the ice sheet is deposited on the ice through precipitation. A much more difficult problem is ascertaining how the Antarctic climate will change!

References

Trupin, A. S., The effect of polar ice on the Earth's rotation and gravitational potential, Geophys. J. Int., 113, 273, 1993.

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